[Label] Add Labels module to tmail-flutter

[dab246]

Desc

• [x] Add Label capability, Label object
• [x] Implement Label/get, Label/set, Label/changes
• [x] Write unit test

Docs

https://github.com/linagora/tmail-backend/blob/master/docs/modules/ROOT/pages/tmail-backend/jmap-extensions/jmapLabels.adoc